1. Field of the Invention
The invention relates generally to high speed steels and, more particularly, to a carburizable high speed steel having low carbon and chromium levels for use as roller bearings, taps and other applications where hardness and fracture resistance are required.
2. Description of the Related Art
It is well-known that stress state on or below a bearing race subjected to alternating contact loads can have a major influence on service life. Most tapered roller bearings are made from carburizing grades of steels. When components made from these grades are carburized and heat treated, some of the advantages realized relative to through hardened components are: fewer quench cracks in heat treating, a decreased sensitivity to grinding injury, and improved toughness or resistance to catastrophic failure which provide a more reliable product exhibiting fewer in-service problems.
For carburized components, the compressive residual stresses are developed during heat treating. The absorption of carbon into a component during carburizing creates a carbon gradient wherein the carbon level is highest near the surface and decreases as the distance away from the surface increases. Thus, the core of the component contains the nominal carbon content of the alloy. When steel components are quenched from the austenitizing temperature, martensite is formed. The transformation of austenite to martensite is accompanied by a volume expansion which is directly proportional to the carbon content of the alloy. When quenched, the surface of a component cools more rapidly than the inner portion of a component. In addition, the Ms temperature (the temperature at which austenite transforms to martensite) decreases with increasing carbon content. Thus, for a carburized component, relative to the core, the surface or so-called xe2x80x9ccasexe2x80x9d transforms to martensite at a lower temperature than would occur for a component of uniform composition. Consequently, these two effects operating in unison cause a relatively high compressive residual stress to be formed on the surface layer or case. Comprehensive details of this type of processing are contained in xe2x80x9cCarburizing and Carbonitridingxe2x80x9d, American Society for Metals, Materials Park, Ohio (1997).
This surface residual stress effect does not occur in components having a uniform composition, i.e., non-carburized components. The enhanced performance created by compressive surface residual stresses has resulted in processes being developed to carburize high carbon bearing alloys, such as that of U.S. Pat. No. 4,191,599 granted Mar. 4, 1980. The ""599 patent discloses the carburizing of high carbon steels such as 52100 and M50 in carburizing atmospheres containing higher carbon potentials than used for standard carburizing steels. The carbon gradients produced lead to reasonable surface compressive residual stresses when these steels are quenched and tempered.
Another factor where compressive residual surface stresses are beneficial involves the press fitting of bearing components onto shafts. It is well-known that press fitting of bearings on shafts can create a tensile stress on the bearing. It has been demonstrated that the press fitting of through hardened AISI 52100 steel definitely has an adverse effect on fatigue life. However, similarly, press fit bearings fabricated from carburized AISI 8620 were found to perform satisfactorily. It was concluded that under press-fitting conditions, carburized AISI 8620 steel had superior fatigue characteristics compared to AISI 52100. This work was reported by T. E. Hustead, xe2x80x9cConsideration of Cylindrical Roller Bearing Load Rating Formulaxe2x80x9d, SAE Preprint 569A (Sept. 1962).
By way of example, consider a LM12749 bearing cone inner race made from carburized 8119 steel compared with the same inner race made from a through hardened 1.0% carbon 46100 steel having a uniform carbon gradient (uncarburized). The compressive stresses in the carburized bearing cone vary from xe2x88x9248.1 ksi on the surface to xe2x88x9222.6 ksi at a depth of 0.030xe2x80x3 below the surface. For the bearing cone made from the through hardened 46100 high carbon alloy steel, the stress from the surface to 0.030xe2x80x3 below the surface was, at most, only xe2x88x923.8 ksi.
While carburized bearings fabricated from low carbon alloy steels have better properties than through hardened bearings fabricated from high carbon alloy steels, neither of these types of alloys performs well at continuous temperatures in excess of 400xc2x0 F. Furthermore, brief exposures to temperatures of 500xc2x0 F. or greater can significantly soften components manufactured from most alloy steels. In demanding applications such as jet engine main bearings, high speed steels (sometimes referred to as xe2x80x9cHSSxe2x80x9d) such as M50 are selected. High speed steels have higher compressive yield stresses than alloy steels. The high compressive yield stresses of these steels are a direct result of the high carbon content of the HSS alloys and a presence of alloying elements such as chromium, molybdenum, vanadium and tungsten.
The heat treatments used for high speed steels are different from the heat treatments used for alloy steels. For example, a typical heat treating cycle for an alloy steel such as AISI 4340 would be to austenitize the material at 1550xc2x0 F. until the entire component was equilibrated for one hour at the austenitizing temperature. The material would then be rapidly removed from the furnace and quenched into oil. After the material cooled to approximately 150xc2x0 F., it would be removed from the quench bath. The alloy would then be tempered for approximately two hours at a temperature of less than 1320xc2x0 F. For maximum hardness and strength, the alloy would be tempered at or below 350xc2x0 F. However, if toughness was important, a tempering temperature of 1150xc2x0 F. would be selected. For a bearing alloy such as AISI 52100, the austenitizing may be 1525xc2x0 F. After quenching, a tempering temperature of approximately 350xc2x0 F. would be used. Low temperature tempering would be used for any bearing fabricated from an alloy steel. This would ensure that the resulting component would be hard and have as high a compressive yield stress as possible. Tempering temperatures exceeding 350xc2x0 F. will lower the hardness and, consequently, the compressive yield stress of bearings made from through hardened steels.
For all alloy steels, after being austenitized and then oil quenched, increasing the tempering temperature is found to decrease the alloy""s hardness. Steels having this type of tempering response are referred to as xe2x80x9cclass 1xe2x80x9d types of steels, depicted in FIG. 1.
The heat treating procedures used for high speed steels typically begin with a preheat of approximately 1450xc2x0 F. to 1550xc2x0 F. Components fabricated from HSS are equilibrated at the preheating temperature for at least one hour. Following the preheat, high speed steel alloys are then quickly placed in an austenitizing furnace that is at a higher temperature. Depending on the alloy, the high austenitizing temperature may range from 2000xc2x0 F. to 2125xc2x0 F. The HSS components are only held at the austenitizing temperature for a brief amount of time, say, 3 to 10 minutes. Following austenitization, the material is quenched into a salt bath at 1000xc2x0 F. After equilibrating in the salt bath, the components are allowed to air cool to at least 150xc2x0 F. If an oil quench is employed, the material should be removed when it reaches 900xc2x0 F., after which, cooling to 150xc2x0 F. in still air is recommended.
Following quenching, high speed steel alloys contain untempered martensite, alloy carbides and retained austenite. Tempering HSS must accomplish two things. The martensite needs to be tempered, and the retained austenite has to be transformed to martensite. The general procedure employed for tempering high speed steels is to heat the alloys to approximately 1000xc2x0 F. for two hours and then air cool to room temperature. The cycle is then repeated one more time. Most high speed steels show xe2x80x9cclass 3xe2x80x9d tempering response, of the type depicted in FIG. 1. When the appropriate tempering temperature is found, the hardness after the tempering cycles is actually greater than the hardness immediately after quenching for HSS alloys.
The material and chemical transformations occurring during the heat treating of high speed steels are much more complex than the transformations that occur in alloy steels. A typical HSS alloy contains from 0.80% to 1.40% carbon. In addition, up to 25% alloy elements may be present. The primary alloying elements are typically a combination of Cr, Mo, V and W. Lesser amounts of Co, Si and Cb may occasionally be present. After these alloys are cast, hot rolled and then annealed, the microstructure consists of low carbon iron, ferrite and a large volume fraction of alloy carbides.
The alloy carbides in high speed steels are generally composed of a combination of alloy elements and carbon; hence, the designation MxCy is used. M represents a metal atom and C designates carbon. X corresponds to the number of metal atoms in the carbide and Y is the number of carbon atoms, respectively. Typical carbides in the annealed high speed steels are MC, M6C and M23C6 types.
When the annealed alloy is preheated to 1550xc2x0 F., the ferrite transforms to austenite, and some of the alloy carbide may dissolve. When the steel is placed in the austenitizing furnace where the temperature is 2050xc2x0 F. or greater, all the M23C6 dissolved. As much as 50% of the M6C and the MC may dissolve at the high austenitizing temperature. As the carbides dissolve, the carbon is dispersed in the austenite matrix. When the alloy is quenched and then cooled to 150xc2x0 F. or less, most of the high carbon austenite transforms to martensite. Some of the austenite is retained, and the carbides that did not dissolve remain. The carbides present are MC and M6C types. At this stage in heat treating, the hardness of the alloy is high. Depending on the total alloy content, the hardness often exceeds 60 HRC (732 KHN).
Tempering high speed steels to temperatures up to 800xc2x0 F. may slightly decrease the hardness of the alloy. However, tempering temperatures near 1000xc2x0 F. increase the hardness of these (class 3) steels, FIG. 1. This phenomenon is referred to as secondary hardening. Two processes are occurring in this temperature range: (1) retained austenite is transformed to martensite; and (2) very small alloy carbides such as Mo2C, W2C and VC are formed.
The high hardness of high speed steels, as well as their resistance to softening at elevated temperatures, is primarily due to the phenomenon of secondary hardening. The formation of the small alloy carbides is primarily responsible for the excellent hot hardness these alloys exhibit.
As increased demands were placed upon bearings used in aircraft engines, M50 high speed steel was selected for applications requiring high temperature service. This alloy achieves its maximum hardness by the phenomenon of secondary hardening. Hence, M50 has good strength at elevated temperatures. The nominal composition of M50 is 0.80% C, 4.10% Cr, 4.25% Mo and 1.00% V. Secondary hardening in this alloy is primarily caused by Mo and V.
The major disadvantage of M50 or other high speed steels is that the relatively high carbon and alloy content of the alloy greatly decreases its fracture resistance or toughness. Considering previous knowledge of the inherent benefits of using carburized components, a low carbon version of M50 was developed. The low carbon variety was named M50 Nil; its nominal composition is: 0.13% C, 4.20% Cr, 3.40% Ni, 4.25% Mo and 1.2% V. The low carbon nickel added variant of M50 has excellent fracture toughness. Furthermore, since carbon is added to the case by a gas metal reaction, the carbides formed during carburizing are smaller than the carbides in wrought M50. The absence of large carbides is beneficial to rolling contact fatigue life.
There is, however, a major disadvantage associated with M50 Nil. Since the alloy contains 4.2% Cr, it is difficult to carburize and, therefore, components fabricated from M50 Nil must be pre-oxidized prior to being carburized. This step creates additional expenses and problems for bearing manufacturers using M50 Nil. Vacuum plasma carburizing can be used on non-oxidized M50 Nil, but this processing is very expensive when compared to standard gas carburizing.
The presence of approximately 4% chromium in most high speed steels may, in part, be the result of early developments with these alloys and their processing. During the preliminary development of these grades of alloys, it was noted that 4% chromium represented the best compromise between hardness and toughness for high speed steels. It should be noted, however, that when compared to alloy steels, the toughness of any HSS is, at best, very poor. While chromium is mainly responsible for the great hardenability of these alloys, this property is only of importance in components having large cross-sectional areas. It is believed that chromium in the matrix increases the difficulty of precipitation and coalescence of the carbide involved in the secondary hardening phenomena. However, chromium alone does not greatly contribute to improvements in hot hardness. In machining tests, less than 4% Cr has shown to decrease cutting efficiency. In the early development of high speed steels, chromium was found to reduce the oxidation and scaling of these alloys during heat treatment. While this factor may have been important in the 1940""s, with today""s modern furnaces and rectified salt baths, oxidation during heat treating can easily be prevented. It is of even greater interest to consider that the resistance to oxidation caused by chromium was considered beneficial, but this same property is what makes M50 Nil difficult to carburize.
A careful analysis of the benefits that have been achieved in using high speed steels in bearing applications, coupled with a recognition of the limitations experienced by these alloys and M50 Nil, form the genesis of the present invention. Ideally, a bearing alloy for high temperature applications should possess the following properties:
The present invention relates to low carbon high speed steels that can be easily carburized using conventional processes employed for standard alloy steels such as 8620, 8720, 4320 or 3311. This family of alloys contains less carbon than is in a standard grade of high speed steel. Preferably, alloys according to the present invention contain less than 0.40 wt. % carbon. In addition, the chromium content of the alloys is less than 2 wt. % and, more preferably, the chromium content is less than 1.5 wt. %. The low chromium content is a critical factor in enhancing the ease of carburizing these steels. In order to obtain the properties described herein, the sum of one or more of (Mo+V+Co+W), including the Cr content, is equal to or greater than 7.5 wt. % ranging up to 35%. As with all steels, the majority of the alloy is composed of iron. The selection of the alloy elements and their effect on properties are listed below:
The present invention also is directed to a method for making a modified high speed steel possessing a surface hardness of 60 HRC or greater, containing less than 10% retained austenite and a fracture resistant core. The method comprises the steps of:
(a) providing an alloy consisting essentially of in % by weight up to 0.4 wt. % C, less than 2% Cr, one or more alloy constituents selected from the group consisting of W, Mo, Co, V in an aggregate amount including the Cr content of at least 7.5% up to about 25 or 35%, 0.10-0.60% Mn, 0.03 max % P, 0.15-0.65% Si, up to 2% Ni, and balance Fe plus incidental impurities;
(b) subjecting the steel to a carburizing treatment at approximately 960xc2x0 C. without any oxidation treatment or heat treatment prior to the carburizing treatment;
(c) quenching the carburized steel;
(d) preheating the quenched carburized steel to 870xc2x0 C., and then austenitizing the steel at temperatures ranging from 1125xc2x0 C. to 1225xc2x0 C.;
(e) quenching the austenitized steel; and
(f) tempering the quenched, austenitized steel, preferably twice, at temperatures up to 550xc2x0 C. followed by air cooling after each tempering treatment.
While the preliminary development of this invention was intended for bearing applications, many other applications are envisioned. For example, taps made from conventional high speed steels are easily broken if too much force is applied to them. A tap made from a carburized HSS possesses much better toughness and fracture resistance than similar high carbon taps.
The surface carbon content of the alloys of the present invention is controlled by the carburizing atmosphere. It is relatively simple to obtain high carbon contents (e.g., up to 1% carbon) on the carburized surface of cutting tools made from the alloys of the present invention. Higher carbon contents make the cutting tools more wear resistant, and the overall toughness of these tools is excellent. Generally, steels with through carbon contents in excess of 1% are very difficult to manufacture in a steel mill. Such high carbon content steels tend to crack after solidifying and are extremely difficult to re-heat and forge or roll.
Since the majority of carbides in the alloys of the invention are formed during carburizing, the overall carbide size distribution is smaller than the carbides in a similar wrought alloy. Furthermore, the number of carbides present prior to carburizing is lower than in a high carbon alloy. Hence, the family of steels of the present invention is easier to machine than standard HSS. The absence of the high volume fraction of carbides provides another advantage to these grades of steels. Higher levels of Mn and S can also be incorporated into these inventive alloys so as to further enhance the machinability of these steels. The following detailed description shows how some selected alloy compositions within the scope of the invention respond to carburizing and heat treating. The accompanying data exemplify the superior physical properties achieved thereby.
A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures.